This invention relates to a gradation conversion system for converting color display data into gradation display data, and more particularly to a gradation conversion system which is suitable for a horizontal display, such as a liquid crystal display or a plasma display, to be used in a personal computer or the like.
2. Description of the Related Art
Generally, in personal computers and workstations, it has been customary to perform color display using a color cathode ray tube (CRT), and many types of application software have been developed on the premise that will be a method of color display.
Meanwhile, accommodating growing needs for the minintuarization of information processing apparatuses, a variety of laptop personal computers are currently being put on the market. In the existing laptop personal computer, to reduce its weight and thickness, a liquid crystal panel or a plasma panel other than the usual color CRT is used as a display. This type of display is hereinafter called a panel display.
As panel displays, some color crystal panels have currently been developed but are still expensive; most of the conventional panel displays are monochromatic. Consequently, when the application software for color display by a color CRT is operated on a laptop personal computer having the panel display, it would make no distinction between colors.
To this end, generally in the panel display, gradation display is performed by varying the lighting time of each individual display dot to represent a difference in color as a difference in gradation. At that time, how to convert color information into gradation information is an important factor in improving display quality.
FIG. 11 of the accompanying drawings is a block diagram showing a typical information processing apparatus incorporated in the conventional gradation conversion method.
In FIG. 11, reference numeral 1 designates a CPU; 2, a CPU bus; 3, a display controller; 5, a display memory for storing the content of display; 7, a pallet circuit for converting logical color data 9 into physical color data 10; 8, a gradation converter circuit for converting the physical color data 10 into gradation data 11; and 12, a panel display.
FIG. 12 is a block diagram showing the display memory 5 and the pallet circuit 7.
In FIG. 12, the pallet circuit 7 is a kind of RAM for storing, as address input, logical color data 9 which is the output data of the display memory 5 and for outputting physical color data 10. The display memory 5 is composed of four planes 0-3 so that logical color data 9 can discriminate at most 2.sup.4 =16 color.
Assuming that 6-bit brightness information for each of the three primary colors RGB (Red, Green, Blue) is stored, the internal RAM of the pallet circuit 7 can output 2.sup.6 .times.2.sup.6 .times.2.sup.6 =262144 colors as physical color data 10. At that time, since the logical color data 9 equivalent to the address input of the pallet circuit 7 is 4-bit data and can represent 16 colors, the pallet circuit 7 may be called a circuit for converting 16 colors into 262144 colors or a circuit for displaying just 16 colors from a total of 262144.
Alternatively if the display memory 5 is composed of eight planes so that the logical color data 9 to be given to the pallet circuit 7 is 8-bit data, the pallet circuit 7 can simultaneously display 256 colors from a total of 262144 colors.
The pallet circuit 7 is also called a color look-up table, which is described in detail in Japanese Patent Publication (KOKAI) No. 54-37943.
The gradations that can be displayed on the panel display 12 are at most sixteen; increasing the number of gradations is difficult technologically and makes it difficult for the user to discriminate gradations from one another.
The gradation converter circuit 8 of FIG. 11 creates, from the physical color data 10 of 262144 colors thus obtained, the gradation data 11 representing 16 gradations. This creating is performed generally by converting RGB information into brightness information Y using the following known equation: EQU Y=0.3R+0.59G+0.11B (1)
Namely the gradation converter circuit 8 creates data of 16 gradations by dividing the brightness information Y, which is obtained by the equation (1), by a threshold value dividing 0-1 into 16 equal parts.
Thus, in the prior art, the color data to be displayed is converted into gradation data to display a difference in color as a difference in gradation on the panel display 12.
In personal computers and the like, the purposes of color display can be divided chiefly into the following two areas:
The first purpose is to show a distinction between letters or regions by a difference in color. Typical examples of this purpose are to display a text, simple graphics, etc. Generally, in this case, 16 colors as shown in FIG. 13 are used; these 16 colors are hereinafter called 16 basic colors. FIG. 13 shows set values of the pallet circuit 7, and brightness and gradation values calculated using the equation (1).
The second purpose is to express the colors themselves. Typical examples of this purpose are to display a natural image, high resolution computer graphics, etc. Generally, in this case, 256 or more colors are used.
In the first purpose, when performing gradation conversion of color display data into monochromatic data, it is preferable to convert different colors into different gradations rather than to express the brightnesses of colors exactly. However, regarding the gradation values shown in FIG. 13, green and bright blue are equal to each other, and yellow and bright purple are equal to each other. By the converting method using the equation (1), it is impossible to convert the 16 basic colors into sixteen different gradations. Therefore, in the case of the first purpose, it is preferable to associate each of the 16 colors with one of 16 gradations.
In the second purpose, to for obtaining a natural monochromatic image, it is necessary to convert the brightnesses of colors into gradations with fidelity, so it is preferably to perform conversion using the equation (1).
Conventionally, in a personal computer capable of displaying only 16 colors even using a color CRT, since a gradation value is obtained by associating one color with one gradation, the 16 basic colors can be converted into 16 gradations.
However in personal computers capable of displaying 16 or 256 colors of 260,000 colors on a color CRT by using a pallet circuit, it is a common practice to perform the gradation conversion using the equation (1). In such a personal computer, if a text or the like using only 16 basic colors is displayed in graduation on a panel display, displaying in 16 different gradations would be difficult to achieve.
Japanese Patent Laid-Open Publication (KOKAI) No. 1-118191 discloses a system which counts the number of colors used in a single screen and performs gradation conversion into gradation codes of constant space according to the number of colors in use.
According to this prior system, in the case where four colors are in use, for example, the gradation codes of four gradations of constant space are outputted from 16 gradations corresponding to the 16 basic colors, irrespective of their physical color data. Therefore, even though the physical data of four colors are contiguous to one another, it is possible to automatically perform gradation conversion into gradation codes easiest to see.
However, in this prior system, it is impossible to perform gradation conversion with fidelity to the brightness of the colors.